A DROP INDEX statement removes an existing index. If the index was not given a name during
creation, the index name is <table_name>_<column_name>_idx. If the index does
not exists, the statement will return an error, unless IF EXISTS is used in which case the
operation is a no-op. You can use dot notation to specify a keyspace for the index you want
to drop: keyspace name followed by a period followed the name of the index. Cassandra drops
the index in the specified keyspace, but does not change the current keyspace; otherwise, if
you do not use a keyspace name, Cassandra drops the index for the table within the current
keyspace.